The art recognizes a variety of factors that affect a polymeric scaffold's/stent's ability to retain its structural integrity when subjected to external loadings, such as crimping and balloon expansion forces. According to the art, characteristics differentiating a polymeric, bio-absorbable scaffolding of the type expanded to a deployed state by plastic deformation from a similarly functioning metal stent are many and significant. Indeed, several of the accepted analytic or empirical methods/models used to predict the behavior of metallic stents tend to be unreliable, if not inappropriate, as methods/models for reliably and consistently predicting the highly non-linear behavior of a polymeric load-bearing portion of a balloon-expandable scaffold (hereinafter “scaffold”). The models are not generally capable of providing an acceptable degree of certainty required for purposes of implanting the scaffold within a body, or predicting/anticipating the empirical data.
Polymer material considered for use as a polymeric scaffold, e.g. poly(L-lactide) (“PLLA”), poly(L-lactide-co-glycolide) (“PLGA”), poly(D-lactide-co-glycolide) or poly(L-lactide-co-D-lactide) (“PLLA-co-PDLA”) with less than 10% D-lactide, and PLLD/PDLA stereo complex, may be described, through comparison with a metallic material used to form a stent, in some of the following ways. A suitable polymer has a low strength to weight ratio, which means more material is needed to provide an equivalent mechanical property to that of a metal. Therefore, struts must be made thicker and wider to have the required strength for a stent to support lumen walls at a desired radius. The scaffold made from such polymers also tends to be brittle or have limited fracture toughness. The anisotropic and rate-dependant inelastic properties (i.e., strength/stiffness of the material varies depending upon the rate at which the material is deformed) inherent in the material, only compound this complexity in working with a polymer, particularly, bio-absorbable polymer such as PLLA or PLGA.
One challenge to a scaffold/stent is uniformly crimping to a balloon and uniform expansion of the scaffold when the balloon is inflated. In the case of a polymeric scaffold, problems arise where, on the one hand, the scaffold cannot be crimped to the desired size without introducing structural failure, i.e., fracture, or excessive cracking, either in the crimped state or when expanded from the crimped state by a balloon. On the other hand, a scaffold can be crimped and deployed, yet deploys with non-uniformity in its deployed state. In these cases the scaffold is susceptible to acute or fatigue failure as the irregularly-deployed rings and/or cells, loaded beyond their design limits as a consequence of the non-uniform deployment, have a reduced acute or fatigue life within the vessel.
Additionally, the retention force keeping a crimped scaffold on a delivery balloon during transit through tortuous anatomy is sometimes not sufficiently high to preclude pre-mature dislodgment of the scaffold from the balloon. If the scaffold is not held on the balloon with sufficient force, e.g., as where there is recoil in the scaffold following crimping or the coefficient of friction between balloon and scaffold is too low, the scaffold can become separated from the balloon as the catheter distal end flexes and/or impinges on walls of the delivery sheath. For a metallic stent, there are several well-known approaches for increasing the retention of the stent to a balloon during transit to the target site. However, methods proposed thus far for retaining the scaffold on a balloon are in need of improvement, or inappropriate for a polymer scaffold.
In one example of a method for crimping a metallic stent to a delivery balloon, the stent is placed in a crimper and the temperature elevated to facilitate greater compliance in the balloon material to allow material to extend between gaps in the stent struts. Additionally, balloon pressure is maintained while the stent is being crimped to increase stent retention to the balloon. After an initial pre-crimp, the stent is placed on the delivery balloon and allowed to slightly recoil under balloon pressure and while the stent has an elevated temperature. After this step, the stent is crimped onto the balloon while the balloon is pressurized. The stent is cycled through larger and smaller diameters. Additionally, balloon pressure may be supplied in bursts or held constant during these crimping steps. Further details of this process may be found in U.S. application Ser. No. 12/895,646 filed Sep. 30, 2010.
The art previously devised methods for retaining a balloon-expanded polymer scaffold on a delivery balloon. In one example, the scaffold is crimped to the delivery balloon at a temperature well below the polymer's TG. Then the scaffold, disposed between ends of the balloon, is thermally insulated from the balloon's ends. The ends of the balloon are then heated to about 185 degrees Fahrenheit to expand the diameter of the balloon material at its ends. The expanded balloon ends form raised edges abutting the scaffold ends to resist dislodgment of the scaffold from the balloon. In one example, this process provided a retention force of about 0.35 lb. for a Poly (L-lactide) (PLLA) scaffold crimped to a polymide-polyether block co-polymer (PEBAX) balloon. An example of this process is disclosed in U.S. Pat. No. 6,666,880.
Another example of a polymer scaffold crimping is found in U.S. Pat. No. 8,046,897, which has a common inventor with the present application. According to the '897 patent the balloon is inflated, or partially inflated before crimping. The scaffold is placed on the balloon. The crimping may take place at elevated temperatures, e.g., 30-50 degrees Celsius.
A film-headed crimper has been used to crimp stents to balloons. Referring to FIG. 7A, there is shown a perspective view of a crimping assembly 20 that includes three rolls 123, 124, 125 used to position a clean sheet of non-stick material between the crimping blades and the stent prior to crimping. For example, upper roll 125 holds the sheet secured to a backing sheet. The sheet is drawn from the backing sheet by a rotating mechanism (not shown) within the crimper head 20. A second sheet is dispensed from the mid roll 124. After crimping, the first and second (used) sheets are collected by the lower roll 123. As an alternative to rollers dispensing a non-stick sheet, a stent may be covered in a thin, compliant protective sheath before crimping.
FIG. 7B illustrates the positioning the first sheet 125a and second sheet 124a relative to the wedges 22 and a stent 100 within the aperture of the crimping assembly 20. As illustrated each of the two sheets are passed between two blades 22 on opposite sides of the stent 100 and a tension T1 and T2 applied to gather up excess sheet material as the iris of the crimping assembly is reduced in size via the converging blades 22.
The dispensed sheets of non-stick material (or protective sheath) are used to avoid buildup of coating material on the crimper blades for stents coated with a therapeutic agent. The sheets 125a, 124a are replaced by a new sheet after each crimping sequence. By advancing a clean sheet after each crimp, accumulation of contaminating coating material from previously crimped stents is avoided. By using replaceable sheets, stents having different drug coatings can be crimped using the same crimping assembly without risk of contamination or buildup of coating material from prior stent crimping.
There is a continuing need to improve upon methods for crimping a medical device and, in particular, a polymer scaffold to a delivery balloon in order to improve upon the uniformity of deployment of a polymer scaffold from the balloon, to increase the retention force between scaffold and balloon, and to obtain a minimal crossing profile for delivery of the scaffold to a target site.